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Convert AC to DC

Bobb FwedBobb Fwed Posts: 1,119
edited 2009-08-02 00:18 in Propeller 1
I need to convert a 120V AC to 12V DC. I have a Zettler AHI01024 transformer (120/240V to 24/12V). It has has 4 connectors on each side of the transformer. I also have a common mode inductor. Attached is a couple guess as to how to wire it up (based on some random stuff on the net). I am looking to provide non-isolated DC power. What is the best way to provide stable power that is the negative side of the DC output is grounded (directly to earth ground. I am regulating the power down to a lower voltage (that 12V) with a linear/LDO regulator. Maybe if I use the common on the regulator to reference the input ground (I think that will work either)....
Am I on the right path? Or am I going to fry my transformer. What is the best way to supply grounded DC power from a AC power source?

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April, 2008: when I discovered the answers to all my micro-computational-botherations!
wiring.jpg
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Comments

  • Bobb FwedBobb Fwed Posts: 1,119
    edited 2009-07-31 23:08
    I just realized this has nothing to do with a propeller, but by habit I have posted it in the Propeller forum. Here is why it is applicable to a propeller: I will be powering a propeller (among other things) with this setup.
    Some additional information: I have tested the second diagram in that image. I used 2 6.8µF caps (tied to AC ground), 16mH common mode inductor, and a 1000µF along with the load. No matter how much load I gave it (I tested between 1000-ohm and 20-ohm) it gives about a 1V fluctuation (relative to ground -- not great, but acceptable as it is going to be regulated down). But that wiring setup is not tied to ground.

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    April, 2008: when I discovered the answers to all my micro-computational-botherations!
  • Mike GreenMike Green Posts: 23,101
    edited 2009-07-31 23:25
    The first diagram is a standard bridge rectifier. It's been a long time since I've seen an inductor and capacitors connected the way you have them in the second diagram. I associate that with high voltage power supplies for vacuum tube ham gear, not low voltage computer gear.

    How much current are you drawing? What's the rating of the transformer windings?

    A 1000uF capacitor across the output sounds about right. What's the capacitor's voltage rating?
  • Bobb FwedBobb Fwed Posts: 1,119
    edited 2009-07-31 23:37
    The 1000µF cap is 16V. I am running about 600mA and the transformer is rated for 10VA. So the first setup is fine? I found it odd how hot the transformer was getting and I figured the diode to ground (through the bridge rectifier) may be causing extra load on the transformer. With the first wiring setup I get almost 2V fluctuation at 20-ohm load (changes a lot with change in load).
    But if that shouldn't be bad for anything, then I guess I already have my solution.

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    April, 2008: when I discovered the answers to all my micro-computational-botherations!
  • localrogerlocalroger Posts: 3,452
    edited 2009-08-01 00:42
    @Bobb, have you tried it without parallelling the transformer windings? In my experience you almost never want to do that. Use one input winding and one output winding and see what happens. When you parallel windings, if they have even slightly different characteristics one will want to be at a different voltage than the other, and the different voltages will be shorted because they're parallelled, and even if the voltage difference is very slight huge currents will flow. This is true in both primay and secondary.
  • Mike GreenMike Green Posts: 23,101
    edited 2009-08-01 00:52
    0.6A x 12V = 7.2VA, so you should be ok

    Paralleling these windings should be fine. They're designed to be identical so you can connect them in series for 240VAC or in parallel for 120VAC with double the current handling capacity. Similarly with the secondaries, they're designed to be connected in parallel (for 24VAC) or in series (for 12VAC).

    The transformer should get fairly warm at load. With 20 Ohms' load and probably 14-16VDC output, you're running close to full rating. A bridge rectifier runs hotter than a half-wave rectifier because the bridge is conducting on both half cycles and the extra 1.4V drop across the rectifier takes a little more "juice" from the transformer.
  • localrogerlocalroger Posts: 3,452
    edited 2009-08-01 00:57
    Mike -- I know many transformers are designed to have their windings parallelled, but I've had several real world examples of transformers that did not like this at all, especially low voltage ones where the old high voltage design ways may not be in vogue any more. It's an easy experiment to do and to undo if it's not the problem.
  • PhilldapillPhilldapill Posts: 1,283
    edited 2009-08-02 00:18
    You might just get the secondary windings and run EACH through a bridge rectifier, but have the DC outputs connected in parallel. If there is any imbalance which is causing the heat, this should get rid of it.
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